Growth of Small Cracks and an Evaluation of Low Cycle Fatigue Life

Smooth and through-thickness cracked specimens of JIS S35C medium carbon and SNCM 439 alloy steels, heat-treated in several conditions, were low cycle fatigued. The growth rates of small surface and through-thickness large cracks, da/dN, were analyzed in terms of the cyclic J-integral range, ΔJ, and the strain intensity factor range, ΔKϵ.

The crack growth rates plotted against ΔJ and ΔKϵ are expressed by straight lines on a log-log diagram. The fairly large scatter inherent in a small crack problem hides some difference in the relations of da/dN versus ΔJ and the da/dN versus ΔKϵ plots between different materials. These relations in a small surface crack are slightly different from those in the through-thickness large crack.

The equations derived from combining these two relations and the cyclic stress-strain curve express well the fatigue life curves, of which slopes decrease in the lower plastic strain range in materials having a high strength and a low ductility, as well as the usual Coffin-Manson plot.